However The fact that it wants to save to the library files in MC25 when I press the rewind key is really puzzling.
That is normal. When you do a manual Library Backup in MC, which a Ctrl+Shift+B initiates, MC will propose the last directory used for a Library Backup, or a Library Restore. In your case, this would have been the MC25 directory. Just initiate a manual Library Backup and navigate to the appropriate MC26 directory before saving, and MC will remember that in the future. Library Restores will then default to that location as well.
That is part of the reason that I have set the Library Backup location in Options to a location that is independent of MC version. I do manual Library Backups to the same location, and hence Library Restores also default to the same location. That mixes backups from various MC versions, but that isn't an issue since I never go back more than one or two backups, and MC will load Library Backups from earlier versions no problem.
MC Uses the backup location set in Options for automatic Library Backups.